The reliable transmission of high data rate information through the short hop radio channel (i.e. a wireless channel having receiver-transmitter distance of less than 1 km) remains a complex and elusive engineering problem. The physical nature of the indoor radio channel causes random distortive effects that can render a communications system inoperable. The prior art includes many elaborate schemes that mitigate these effects, however they are generally very complex and unsuitable for applications requiring mobility and/or portability.
Nature has already solved this problem, however, in the form of bats. Bats use echolocation for nighttime navigation and face many of the same distortion effects that radio systems do. Through the process of evolution they have developed powerful probing signals called chirps, which overcome many of these effects.
The distortive effects referred to previously arise largely from the multipath nature of the radio channel, that is, a signal may travel from transmitter to receiver via many different paths. This is in contrast to a wired channel (a telephone line, for instance) in which a signal can only travel along a single path. Multipath occurs because the physical space between the receiver and transmitter is occupied by objects which reflect the radio signal. The reflections create echoes which appear at the receiver as copies of the original transmitted signal. These echoes have independent (i.e. random) amplitudes and delay times with respect to each other. At the receiver they add together to create an unpredictable total signal; this signal may also change with time as objects in the physical environment move about.
When multipath echoes add together as they do in the receiver of a wireless communications system, two main distortive effects can result. First, the addition of the delayed echoes at the receiver stretches out or disperses the original transmitted signal. This dispersion makes the transmitted symbols longer, and if adjacent symbols are not spaced far enough apart in time they can collide and Intersymbol Interference (ISI) can occur. Second, the periodic nature of the transmitted RF carrier along with the random delays of each echo can cause them to add destructively. Under the right conditions, this destructive interference (called fading) between echoes can cause reductions in the received power by a factor of 1000 or more.
The main engineering problem arising from these two effects has to do with their relationship to the transmitted signal's bandwidth. If the transmitted signal has a narrow bandwidth, (i.e. does not cover a large range of transmitted frequencies) the receiver will experience flat fading and the entire signal can sit within a fade for a short period of time. Under these conditions the communication system will not function correctly. This condition can be mitigated by widening the occupied bandwidth of the signal (i.e. so it covers a large range of frequencies). The simplest way to increase the transmitted signal's bandwidth is to increase the transmitted symbol rate. Unfortunately, increasing the symbol rate also reduces the time between symbols increasing the probability that dispersion in the channel will cause adjacent symbols to collide and produce ISI. This presents the dilemma of wireless data transmission: in order to reduce the possibility of flat fading the symbol rate must be increased to the point where ISI can occur.
The art of indoor wireless communications includes techniques that can utilize large bandwidths with respect to their symbol rate; these are commonly referred to as spread spectrum systems. These techniques overcome the paradox between intersymbol interference and flat fading. The two most popular spread spectrum methods, Direct Sequence and Frequency Hopping, both increase the signal's bandwidth by dividing the symbol time into smaller pieces and performing specific operations on each piece. This operation increases the occupied bandwidth of the system by whatever factor the symbol time was divided, thus eliminating flat fading effects. The cost of using spread spectrum is increased system complexity required for encoding, synchronization and decoding of the aforementioned pieces, which must occur at a much higher speed than the symbol rate.